This invention relates to inkjet printers and, more particularly, to a monolithic printhead for an inkjet printer.
The various fully integrated thermal inkjet printheads described in the above-identified applications by Naoto Kawamura et al. include thin film layers containing heater resistors, conductors, and other layers over a silicon substrate. The backside of the substrate is etched completely through (forming a trench), and holes are formed through the thin film layers to allow ink to flow from the backside of the substrate, through the substrate, and into vaporization chambers formed on the top surface of the substrate. Energizing a heater resistor vaporizes a portion of the ink within a vaporization chamber, creating a bubble, which causes a droplet of ink to be ejected out of an associated nozzle in an orifice member formed over the thin film layers. Multiple embodiments were shown in the previous applications. FIGS. 1-3 herein are reproduced from the previous applications to place into context the present improvement over the printheads disclosed in the previous application.
FIG. 1 is a perspective view of one type of inkjet print cartridge 10 which may incorporate the printhead structures described herein. The print cartridge 10 of FIG. 1 is the type that contains a substantial quantity of ink within its body 12, but another suitable print cartridge may be the type that receives ink from an external ink supply either mounted on the printhead or connected to the printhead via a tube.
The ink is supplied to a printhead 14. Printhead 14 channels the ink into ink ejection chambers, each chamber containing an ink ejection element. Electrical signals are provided to contacts 16 to individually energize the ink ejection elements to eject a droplet of ink through an associated nozzle 18. The structure and operation of conventional print cartridges are very well known.
FIG. 2 is a cross-sectional view of a portion of the printhead of FIG. 1 taken along line 2xe2x80x942 in FIG. 1. Although a printhead may have 300 or more nozzles and associated ink ejection chambers, detail of only a single ink ejection chamber need be described in order to understand the invention. It should also be understood by those skilled in the art that many printheads are formed on a single silicon wafer and then separated from one another using conventional techniques.
In FIG. 2, a silicon substrate 20 has formed on it various thin film layers 22. The thin film layers 22 include a resistive layer for forming resistors 24. Other thin film layers perform various functions, such as providing electrical insulation from the substrate 20, providing a thermally conductive path from the heater resistor elements to the substrate 20, and providing electrical conductors to the resistor elements. One electrical conductor 25 is shown leading to one end of a resistor 24. A similar conductor leads to the other end of the resistor 24. In an actual embodiment, the resistors and conductors in a chamber would be obscured by overlying layers.
Ink feed holes 26 are formed completely through the thin film layers 22.
An orifice layer 28 is deposited over the surface of the thin film layers 22 and developed to form ink ejection chambers 30, one chamber per resistor 24. A manifold 32 is also formed in the orifice layer 28 for providing a common ink channel for a row of ink ejection chambers 30. The inside edge of the manifold 32 is shown by a dashed line 33. Nozzles 34 may be formed by laser ablation using a mask and conventional photolithography techniques. Chemical etching may also be used to form the orifice layer.
The silicon substrate 20 is etched to form a trench 36 extending along the length of the row of ink feed holes 26 so that ink 38 from an ink reservoir may enter the ink feed holes 26 for supplying ink to the ink ejection chambers 30.
In one embodiment, each printhead is approximately one-half inch long and contains two offset rows of nozzles, each row containing 150 nozzles for a total of 300 nozzles per printhead. The printhead can thus print at a single pass resolution of 600 dots per inch (dpi) along the direction of the nozzle rows or print at a greater resolution in multiple passes. Greater resolutions (e.g., 1200 dpi) may also be printed along the scan direction of the printhead.
In operation, an electrical signal is provided to heater resistor 24, which vaporizes a portion of the ink to form a bubble within an ink ejection chamber 30. The bubble propels an ink droplet through an associated nozzle 34 onto a medium. The ink ejection chamber is then refilled by capillary action.
FIG. 3 is a cross-sectional perspective view along line 2xe2x80x942 in FIG. 1 illustrating a single ink ejection chamber 40 in another embodiment of a monolithic printhead described in the prior applications.
In FIG. 3, a silicon substrate 50 has formed on it a plurality of thin film layers 52, including a resistive layer and an AlCu layer that are etched to form the heater resistors 42. AlCu conductors 43 are shown leading to the resistors 42.
Ink feed holes 47 are formed through the thin film layers 52 to extend to the surface of the silicon substrate 50. An orifice layer 54 is then formed over the thin film layers 52 to define ink ejection chambers 40 and nozzles 44. The silicon substrate 50 is etched to form a trench 56 extending the length of the row of ink ejection chambers. The trench 56 may be formed prior to the orifice layer. Ink 58 from an ink reservoir is shown flowing into trench 56, through ink feed hole 47, and into chamber 40.
The applications incorporated by reference describe in detail the manufacturing processes for forming the embodiments of FIGS. 2 and 3 and need not be repeated herein. Such processes may use conventional techniques for forming printhead thin film layers.
The thin film layers formed over the substrate in FIGS. 2 and 3 are only on the order of 4 microns thick and, thus, when the underlying silicon is etched away, the thin film (or membrane) is prone to buckling when the trench widths are greater than about 70 microns. Such buckling of unsupported membrane widths greater than 70 microns cause ink drop trajectory errors. Cracks may also be a problem within the membrane shelf and are catastrophic, leading to resistor xe2x80x9copensxe2x80x9d and gross topology changes. These are serious issues needed to be resolved to increase the longevity of these devices.
An additional issue regarding FIGS. 2 and 3 is that there is not satisfactory heat transfer between the heater resistors and the bulk silicon via the membrane at high firing frequencies. This leads to overheating of the membrane. Such overheating of the membrane, and particularly the membrane backside, may heat the ink contacting the backside of the membrane to the point where the ink is vaporized, and bubbles are formed in unwanted areas. These bubbles can cause vapor lock, preventing refill of the firing chambers. One attempted solution was to deposit a layer of metal on the backside of the membrane, but this approach has various drawbacks and is thus not a viable long-term solution.
Accordingly, what is needed is a technique for accurately controlling the width of the backside substrate etching to limit the width of any unsupported membrane to a desired width. It would be further desirable to avoid unsupported membrane widths altogether. What is also desirable is a technique for increasing the heat transfer between the heater resistors and the bulk substrate to prevent the above-described problems from occurring.
We have overcome the above-described problems by using a silicon-on-insulator (SOI) wafer as the starting substrate. In one embodiment, the substrate consists of a relatively thick layer of silicon (e.g., 660 microns) on which is formed a layer of thermal oxide approximately 5,000 Angstroms, on top of which is a thin layer of silicon (e.g., 10 microns). Thin film layers, including the heater resistors, are formed over the thin silicon layer. An orifice layer containing nozzles and vaporization chambers is then formed.
A backside trench is etched into the thick layer of silicon using a TMAH etch, and the oxide acts as an etch stop. An etch step using, for example, BOE, then removes the exposed portion of the thermal oxide layer between the two silicon layers. A second TMAH etch is then performed to etch through the thin remaining silicon layer to form ink channels completely through the SOI wafer leading to the vaporization chambers.
The oxide layer in conjunction with the thin silicon layer provides much greater control over the width of the trench so as to provide a very predictable silicon membrane beneath the heater resistors. This silicon membrane not only prevents buckling but also acts to increase the heat transfer between the heater resistors and the bulk silicon.
In another embodiment, an SOI wafer is not used, and the disclosed process leaves a thin silicon membrane remaining beneath the heater resistors.